iCloud & iCloud Photos · iCloud Drive basics
How do I turn on iCloud Drive on my Mac?
Short answer: Open System Settings → your name → iCloud and turn on iCloud Drive (sometimes labelled Sync this Mac). If it's missing from Finder's sidebar, tick it in Finder → Settings → Sidebar. Storage Studio then finds it automatically.
Open System Settings, click your name at the top, then iCloud, and turn on iCloud Drive (on some versions the switch is labelled Sync this Mac). After a moment iCloud Drive appears in Finder's sidebar.
If it doesn't appear in Finder, open Finder → Settings → Sidebar and tick iCloud Drive.
Once it's on, Storage Studio finds it automatically — there's nothing to add or sign in to in Storage Studio. See does Storage Studio work with iCloud Drive.